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Articles
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Séminaire PMMH - Karen Mulleners (EPFL)
21 avrilVendredi 3 mai de 11h00 à 12h00 - Salle réunion PMMH 1Getting smarter overnight : how automated experiments help unwind unsteady vortex-dominated flows
Abstract : Typical unsteady vortex-dominated flows like those involved in bio-inspired propulsion, unsteady airfoil separation, and vortex-induced vibrations can be prohibitively expensive to simulate and impossible to measure comprehensively. They are inherently non-linear, often involve moving boundaries, high-dimensional parameter spaces, and multiscale flow structures. The classical way to get around these challenges has been to reduce the experimental complexity by using canonical motions (e.g. ramp-up or sinusoidally pitching motions) or simplified unsteady inflow conditions (e.g. one-minus-cosine or trapezoidal gust shapes). In our lab, we design automated experiments that can run continuously and autonomously such that we can explore and exploit higher-dimensional parameter spaces that cover more realistic and technically relevant unsteady conditions compared to what is traditionally feasible when conducting supervised canonical motion experiments. This approach give us the ability to derive more robust and generalizable models and control solutions while still discovering rare and extreme events. Our recent experiments allowed us to uncover flapping wing kinematics that maximize lift and efficiency, to optimize blade pitching kinematics that improve the power production of vertical axis wind turbines, and to gain insight into the influence of morphology on the forces of thin flexible objects. Once the experiments are started, the experimentalists' input is minimized while the potential for scientific discovery is maximized.
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Séminaire PMMH - François Petrelis (ENS)
5 avrilVendredi 31 mai de 11h00 à 12h00 - Salle réunion PMMH 1Earthquake statistical properties : an explanation for the distribution of magnitude and for the existence of aftershocks
Earthquakes in nature follow several statistical properties. In particular, the distribution of energy released by an earthquake (Gutenberg-Richter's law) and the frequency of aftershocks after a large event (Omori's law) are both power-laws.
By studying several earthquake models, we have shown that the Gutenberg-Richter law results from the spatial distribution of the stress field. This field is self-similar at large scale and for two dimensionnal systems can be modelled as a random surface. Using this analogy, a series of predictions is made that includes the Gutenberg-Richter law and the value of its exponent (so called b-value) together with the existence of aftershocks and their temporal distribution following Omori's law.
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Séminaire PMMH - Benjamin Guiselin - ENS Lyon
19 marsVendredi 12 avril de 11h00 à 12h00 - Salle réunion PMMH 1Emergence of spontaneous collective oscillations in dense Human crowds
Massive crowd gatherings form some of the most dangerous and unpredictable environments [1]. However, we lack quantitative characterizations of their dynamics and the heuristic principles used to explain and predict their motion remain elusive. In this talk, I will present our analysis of the dynamics of thousands of packed individuals in a model system, namely, the opening ceremony of the festival of San Fermín in Pamplona, Spain [2]. This analysis reveals that at extreme densities crowds experience self-sustained oscillatory flows, which echo the correlated orbital motion of hundreds of individuals in the absence of any external guidance. I will then detail how the combination of mechanics and symmetry principles has allowed us to establish a robust predictive model of dense crowds inferred from our measurements to elucidate this emergent chiral dynamics. In particular, we establish that the self-organization of crowds into macroscopic oscillators originates from transverse frictional forces between the crowd and the ground.
[1] D. Helbing, A. Johansson, and H. Z. Al-Abideen. ``Dynamics of crowd disasters : An empirical study''. Physical Review E, 75(4), 046109 (2007).
[2] F. Gu*, B. Guiselin*, N. Bain, I. Zuriguel, and D. Bartolo. ``Emergence of collective oscillations powered by odd friction in massive crowds''. Submitted (2024).
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Séminaire PMMH - Olivier Plé (LOCIE-CNRS)
19 marsVendredi 5 avril de 11h00 à 12h00 - Salle réunion PMMH 1Rammed earth : A complex medium
Rammed earth is a complex material which is attractive nowadays in construction because of its small environmental footprint. However, this material has specific characteristics, especially : its sensitivity to water, its low mechanical static capacity but a fairly good seismic performance, its high thermal conductivity which cannot guarantee sufficient winter comfort, its high heterogeneity which makes it a size effect material….. In short, a complex, unsaturated heterogeneous material with numerous couplings.
In this presentation I will try to show the complexity of the material and how physics helps us to understand for a better use in construction. -
Séminaire PMMH - Alex Hansen (NTNU Norvège)
19 marsVendredi 29 mars de 11h00 à 12h00 - Salle réunion PMMH 1Séminaire d'Alex Hansen (NTNU, Trondheim, Norvège)
The co-moving velocity, a new concept in immiscible two-phase flow in porous media
Alex Hansen
PoreLab, Department of Physics, NTNU, Trondheim, NorwaySince 1936, relative permeability theory has been the leading description of immiscible two-phase flow in porous media at scales much larger than the pore scale. Central to this theory are the two relative permeabilities, one for each fluid, which measures the reduction of mobility each fluid experiences due to the presence of the other fluid. The theory assumes the two relative permeabilities to be functions of the saturation (i.e., relative concentration) alone. When there are saturation gradients present, a third parameter comes into play, the capillary pressure. This is also assumed to depend on the saturation alone.
Such a theory is clearly quite limited in that it makes many strong assumptions. Yet, it is essentially the only one that is used for practical calculations. Can one do better ? That is, come up with a theory that is closer to the physics that is going on and at the same does not drown in complexity ? My answer is yes [1-7]. The aim of this talk is to describe this new theory. I will focus on a new velocity that pops up, namely the co-moving velocity. This velocity has remarkable properties that hints at something deeper which is yet to be uncovered.
References
[1] A. Hansen, S. Sinha, D. Bedeaux, S. Kjelstrup, M. Aa. Gjennestad and M. Vassvik, Relations between seepage velocities in two-phase flow in homogeneous porous media, Transp. Porous Med. 125, 565 (2018) ; doi:10.1007/s11242-018-1139-6.
[2] S. Roy, S. Sinha, and A. Hansen, Flow-area relations in immiscible two-phase flow in porous media, Front. Phys. 8, 4 (2020) ; doi:10.3389/fphy.2020.00004.
[3] S. Roy, H. Pedersen, S. Sinha, and A. Hansen, The co-moving velocity in immiscible two-phase flow in porous media, Transp. in Porous Media, 143, 69 (2022) ; doi:10.1007/s11242-022-01783-7.
[4] A. Hansen, E. G. Flekkøy, S. Sinha, and P. A. Slotte, A statistical mechanics for immiscible and incompressible two-phase flow in porous media, Adv. Water Res., 171, 104336 (2023) ; doi:10.1016/j.advwatres.2022.104336.
[5] H. Pedersen and A. Hansen, Parametrizations of immiscible two-phase flow in porous media, Front. Phys. 11, 1127345 (2023) ; doi:10.3389/fphy.2023.1127345.
[6] F. Alzubaidi, J. E. McClure, H. Pedersen, A. Hansen, C. F. Berg, P. Mostaghimi and R. T. Armstrong, The impact of wettability on the co-moving velocity of two-fluid flow in porous media, arXiv:2309.0036.
[7] J. Feder, E. G. Flekkøy, and A. Hansen, Physics of Flow in Porous Media, (Cambridge University Press, 2022).